This invention relates generally to visibly transparent infrared reflecting interference filters, and more particularly, to a durable low-emissivity filter.
The use of transparent panels in buildings, vehicles and other structures for controlling solar radiation is quite prevalent today. The goal of solar control is to transmit light while excluding much of the solar energy, thus decreasing the amount of air condition or cooling required, and conserving energy. In addition, modified glass as a structural material provides the color flexibility architects desire.
Various processes have been employed to alter the optical properties of these panels, including coating glass or plastic substrates by various techniques such as electrolysis, chemical vapor deposition and physical vapor deposition, including sputtering with planar magnetrons. For instance, thin metal films have been deposited on glass or plastic to increase the reflectance of solar radiation. Windows deposited with a multi-layer dielectric-metal-dielectric coating that exhibits high visible transmittance, and high reflectivity and low emissivity in the infrared range, are even more energy efficient. The index of refraction of the dielectric layer is preferably 2.0 or greater in order to minimize the visible reflectance and enhance the visible transmittance of the window. This dielectric layer which often consists of metal oxide coating also offers additional protection to the fragile metal films. The optical properties of panels can also be modified by altering the composition of the substrate material. Nevertheless, interference filter panels manufactured by the above-described methods have been only partially successful in reflecting solar radiation to the degree required for significant energy conservation. For example, Apfel et al., U.S. Pat. No. 3,682,528, issued Aug. 8, 1972, described an infra-red interference filter with visible light transmission of only approximately 72% and with infra-red transmission of approximately 8%.
Another problem associated with interference filters is their structural integrity, particularly their inability to withstand harsh environmental conditions. Current transparent panels having high reflectivity for solar radiation are unsatisfactory in that the panels are subject to chemical and mechanical degradation.